Remove librustrt, audio Task

librustrt and Task have been removed: https://github.com/rust-lang/rust/pull/19654. This pull request is pretty much only code removal.
Basically, no manual runtime initialization needs to be done when calling Rust from C anymore. It should now "just work".

With all that said and done, I do have some concerns that might be amended in the future:

1. If the Audio callback thread panics, the entire program will abort (because the panic crosses the C -> Rust boundary). This can be unpleasant, as other threads won't be able to gracefully unwind if this happens.

2. In the future, I'd like for the Audio thread to be named in case of a panic (it currently spits out something like `thread "<unnamed>" panicked at "Noes", ...`). There doesn't seem to be a way to set the name of a native thread yet.

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This is based off Gekkio's pull request #238, which hasn't been merged yet. :)
Probably a non-issue, but it might be best to accept their pull request first. I'm not quite sure what happens if a pull request includes someone else's pull request. :/